1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital copying machine, a digital printer, or a digital facsimile, and more particularly to a writing device to write image information on a photoconductive body to form an electrostatic latent image of an image thereupon by scanning luminous flux, emitted by a light source in response to an image signal of the image information, in a main scanning direction on the photoconductive body using a scanning device.
2. Discussion of the Background
A digital image forming apparatus includes a writing device to write image information on a surface of a photoconductive body traveling in a sub-scanning direction to form an electrostatic latent image of an image thereupon by driving a laser light source to emit luminous flux in response to an image signal corresponding to the image information using a write clock and scanning the luminous flux from the light source in a main scanning direction on the photoconductive body using a scanning device such as a rotary polygon mirror. The apparatus includes a developing device to develop the electrostatic latent image on the photoconductive body to a toner image and a transfer device to transfer the toner image on the photoconductive body onto a selected transfer paper.
In such a digital image forming apparatus, a magnification error in the main scanning direction is generally caused by an optical error in the writing device, a write clock error, an expansion or contraction of a transfer paper or other various causes. In order to correct such a magnification error in the main scanning direction caused by the optical characteristic of the writing device, some apparatuses use a variable write clock for driving a light source to emit luminous flux in response to an image signal. In such apparatuses, an expansion or contraction of an image due to an optical error of the writing device is measured and the write clock is adjusted according to a result of the measurement of the error such that the formed image has a predetermined length in the main scanning direction. The write clock adjusted and determined as described above is fixed other than when a further adjustment by a serviceman or other personnel is required.
An exemplary example of a digital image forming apparatus of a digital copying machine is described below. A digital copying machine shown in FIG. 4 has an image reader 11 as an image reading device, a printer section 12 as an image forming section, and an automatic document feeder 13. The automatic document feeder 13 conveys documents which have been set therein one by one onto a contact glass 14 and then discharges the document on the contact glass 14 after reading an image of the document.
The image reader 11 has a first carriage equipped with a light source including a lighting lamp 15 and a reflecting mirror 16 and a first mirror 17, a second carriage equipped with a second mirror 18 and a third mirror 19, a filter 20, a lens 21, and a line sensor 22 including a CCD as an imaging element.
When the image reader 11 reads an image of a document, the image of the document on the contact glass 14 is optically scanned by a travel of the first carriage at a fixed speed and a travel of the second carriage following the first carriage at a half speed thereof while the contact glass 14 is lighted by the lighting lamp 15 and the reflecting mirror 16, by which a reflected light image is formed on the line sensor 22 by the lens 21 via the first mirror 17, the second mirror 18, the third mirror 19, and the filter 20.
The line sensor 22 photo-electrically converts the formed reflected light image of the document into electrical signals and outputs analog image signals, so that the image of the document is read. An ND filter is used as the filter 20 to read a single-color image. After a completion of reading the image of one sheet of the document, the first and second carriages return to a home position.
As the filter 20, it is also possible to use red, green and blue color filters and sequentially change these three color filters in successive image readings so as to obtain red, green, and blue analog image signals sequentially from the line sensor 22. Furthermore, it is possible to use a 3-line CCD having red, green, and blue color filters as the line sensor 22.
The analog image signals from the line sensor 22 are converted to digital image signals by an analog-to-digital converter (not shown), subjected to various image processing, such as conversion processing from multi-level values to bi-level values, gradation processing, magnification processing, editing processing, etc., with an image processing board 23 as an image processing device, and sent to a semiconductor driving board (not shown).
While a photoconductive drum, for example, is used for an image carrier 25 as a photoconductive body in the printer section 12, a photoconductive belt or the like can be used. In a copying operation, the photoconductive drum 25 is driven to rotate by a driving section (not shown), charged uniformly by a charging device 26, and exposed to laser beams by a laser beam scanner 27, by which a latent image is formed on the surface of the photoconductive drum 25. In this operation, the laser beam scanner 27 exposes the photoconductive drum 25 to laser beams from a semiconductor laser (not shown) driven by the foregoing semiconductor driving board in response to the digital image signals from the image processing board 23, and the photoconductive drum 25 travels in the sub-scanning direction in a position where the latent image is written by the laser beam scanner 27, by which the latent image of each single line of the image is formed on the photoconductive drum 25 at an equal interval from each other in the sub-scanning direction. The electrostatic latent image on the photoconductive drum 25 is developed by a developing device 28 so as to be a toner image.
A transfer paper is fed to a registration roller 36 as a selected transfer paper from a feeder selected out of feeders 33 to 35 by a selecting device such as an operating section (not shown), or a transfer paper having a size selected by the selecting device or an automatic paper selecting device (not shown) is fed to the registration roller 36 as a selected transfer paper from a feeder out of the feeders 33 to 35. The transfer paper is sent out by the registration roller 36 at such a timing to register with the toner image on the photoconductive drum 25.
The toner image on the photoconductive drum 25 is transferred to the transfer paper sent out from the registration roller 36 by a transfer device 30 and the transfer paper is separated from the photoconductive drum 25 by a separating device 31. The separated transfer paper is then conveyed by a conveying device 37 toward a fixing device 38. The image on the transfer paper conveyed by the conveying device 37 is fixed by the fixing device 38 and the transfer paper is then ejected onto a tray 39 as a copy. The photoconductive drum 25 is cleaned by a cleaning device 32 to remove residual toner therefrom after the transfer paper is separated from the photoconductive drum 25.
The laser beam scanner 27 includes, as illustrated in FIG. 5, a beam emitting unit 40 including a light source having a semiconductor laser, a collimating lens and an aperture, which are not shown. The semiconductor laser in the beam emitting unit 40 is driven by the foregoing semiconductor driving board in response to digital image signals from the image processing board 23 to emit luminous flux of a laser beam modulated by the digital image signals.
The laser beam from the semiconductor laser is changed to parallel luminous flux by the collimating lens in the beam emitting unit 40 and shaped to luminous flux having a certain shape by passing through the aperture in the beam emitting unit 40. The luminous flux is compressed in the sub-scanning direction by a cylindrical lens 41 and incident on a deflector 42 as a scanning device. A rotary polygon mirror having an accurate polygon shape is used as the deflector 42. The polygon mirror 42 is driven to rotate at a constant speed in a constant direction by a polygon motor 43.
The rotation speed of the polygon motor 43 depends upon a rotation speed of the photoconductive drum 25, a writing speed of the laser beam scanner 27, the number of planes of the polygon mirror 42, and the number of laser beams scanned simultaneously by the polygon mirror 42. The laser beam incident on the polygon mirror 42 from the cylindrical lens 41 is deflected by the polygon mirror 42 so as to be incident on an f.theta. lens 44. The laser beam is scanned in the main scanning direction by a rotation of the polygon mirror 42.
The f.theta. lens 44 converts the light deflected by the polygon mirror 42 so as to scan the surface of the photoconductive drum 25 at equiangular velocity. The laser beam from the f.theta. lens 44 forms an image on the photoconductive drum 25 via a reflecting mirror 45 and a dust-proof glass 46. The f.theta. lens 44 has an inclined surface correction function. The laser beam having passed through the f.theta. lens 44 is reflected by a synchronization detecting mirror 47 outside an image area and guided to a synchronization detecting sensor 48, which detects the laser beam from the synchronization detecting mirror 47 and generates a synchronization detecting signal. The above semiconductor driving board drives the semiconductor laser in the beam emitting unit 40 to emit luminous flux in response to the digital image signals from the image processing board 23 in synchronization with the synchronization detecting signal from the synchronization detecting sensor 48, and the synchronization detecting signal from the synchronization detecting sensor 48 is used as a timing reference to start writing image information of each single line of the image on the photoconductive drum 25 in the main scanning direction.
In the above digital copying machine, when an image is written on the photoconductive drum 25 with laser beams from the laser beam scanner 27, a magnification error in the main scanning direction is caused according to the characteristics of the lens 44 of the laser beam scanner 27.